mod/modregistry: allow RegistryResolver to return ErrRegistryNotFound

When using an explicit `none` registry, we don't want the error
returned by `Resolver.ResolveToRegistry` to count as a hard
error and prevent resolution of other module paths, so
define a new error type and make `modconfig.Resolver.ResolveToRegistry`
return it.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Ica0b210628d337118dd11de166428ada1f33b576
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193764
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/testdata/script/modtidy_nofallback.txtar

@@ -0,0 +1,26 @@
+# Check that we can tidy a module when using a no-fallback
+# "none" registry.
+
+env CUE_REGISTRY=foo.com/bar=$CUE_REGISTRY1,none
+exec cue mod tidy
+exec cue eval .
+cmp stdout want-stdout
+
+-- want-stdout --
+x: "hello"
+-- cue.mod/module.cue --
+module: "main.org@v0"
+language: version: "v0.8.0"
+-- main.cue --
+package main
+import "foo.com/bar/baz@v0"
+
+baz
+
+-- _registry1/foo.com_bar_v0.0.1/cue.mod/module.cue --
+module: "foo.com/bar@v0"
+language: version: "v0.8.0"
+
+-- _registry1/foo.com_bar_v0.0.1/baz/baz.cue --
+package baz
+x: "hello"

cmd/cue/cmd/testdata/script/registry_nofallback.txtar

@@ -4,7 +4,7 @@ env CUE_REGISTRY=foo.com=$CUE_REGISTRY1,none
 cmp stderr expect-stderr
 
 -- expect-stderr --
-import failed: cannot find package "example.com@v0": cannot fetch example.com@v0.0.1: cannot resolve example.com (version v0.0.1) to registry:
+import failed: cannot find package "example.com@v0": cannot fetch example.com@v0.0.1: module not found:
     ./main.cue:2:8
 -- cue.mod/module.cue --
 module: "main.org@v0"

mod/modconfig/modconfig.go

@@ -152,7 +152,7 @@ func (r *Resolver) ResolveToLocation(mpath string, version string) (HostLocation
 	return r.resolver.ResolveToLocation(mpath, version)
 }
 
-// Resolve implements modregistry.Resolver.Resolve.
+// ResolveToRegistry implements [modregistry.Resolver.ResolveToRegistry].
 func (r *Resolver) ResolveToRegistry(mpath string, version string) (modregistry.RegistryLocation, error) {
 	loc, ok := r.resolver.ResolveToLocation(mpath, version)
 	if !ok {
@@ -162,7 +162,7 @@ func (r *Resolver) ResolveToRegistry(mpath string, version string) (modregistry.
 		//
 		// It can also happen when the user has explicitly configured a "none"
 		// registry to avoid falling back to a default registry.
-		return modregistry.RegistryLocation{}, fmt.Errorf("cannot resolve %s (version %s) to registry", mpath, version)
+		return modregistry.RegistryLocation{}, fmt.Errorf("cannot resolve %s (version %q) to registry: %w", mpath, version, modregistry.ErrRegistryNotFound)
 	}
 	r.mu.Lock()
 	defer r.mu.Unlock()

mod/modregistry/client.go

@@ -59,9 +59,16 @@ type Resolver interface {
 	// will hold the prefix that all versions of the module in its
 	// repository have. That prefix will be followed by the version
 	// itself.
+	//
+	// If there is no registry configured for the module, it returns
+	// an [ErrRegistryNotFound] error.
 	ResolveToRegistry(mpath, vers string) (RegistryLocation, error)
 }
 
+// ErrRegistryNotFound is returned by [Resolver.ResolveToRegistry]
+// when there is no registry configured for a module.
+var ErrRegistryNotFound = fmt.Errorf("no registry configured for module")
+
 // RegistryLocation holds a registry and a location within it
 // that a specific module (or set of versions for a module)
 // will be stored.
@@ -104,6 +111,9 @@ func NewClientWithResolver(resolver Resolver) *Client {
 func (c *Client) GetModule(ctx context.Context, m module.Version) (*Module, error) {
 	loc, err := c.resolve(m)
 	if err != nil {
+		if errors.Is(err, ErrRegistryNotFound) {
+			return nil, ErrNotFound
+		}
 		return nil, err
 	}
 	rd, err := loc.Registry.GetTag(ctx, loc.Repository, loc.Tag)
@@ -130,6 +140,10 @@ func (c *Client) GetModule(ctx context.Context, m module.Version) (*Module, erro
 func (c *Client) GetModuleWithManifest(ctx context.Context, m module.Version, contents []byte, mediaType string) (*Module, error) {
 	loc, err := c.resolve(m)
 	if err != nil {
+		// Note: don't return [ErrNotFound] here because if we've got the
+		// manifest we should be pretty sure that the module actually
+		// exists, so it's a harder error than if we're getting the module
+		// by tag.
 		return nil, err
 	}
 
@@ -168,6 +182,9 @@ func (c *Client) ModuleVersions(ctx context.Context, m string) ([]string, error)
 	}
 	loc, err := c.resolver.ResolveToRegistry(mpath, "")
 	if err != nil {
+		if errors.Is(err, ErrRegistryNotFound) {
+			return nil, nil
+		}
 		return nil, err
 	}
 	versions := []string{}